1. Field of Invention
The present invention relates to microelectromechanical system (MEMS) device. More particularly, the present invention relates to a method for fabricating CMOS MEMS device having single crystal structure (SCS) for the MEMS structure.
2. Description of Related Art
MEMS device has been proposed in various applications. However, the MEMS device usually is staying in an open environment. Since the MEMS structure of the MEMS device is sensitive to the environmental air, the operation in open environment may pick up noise. A hermetic MEMS device is then proposed.
A hermetic MEMS package has properties including isolating the MEMS from the outside environment. For example, the damping of air and thermal noise can be avoided by forming a vacuum environment for MEMS. In addition, hermetic camber can protects the MEMS device from EM interference and produce a small size MEMS device with high performance and low cost for popularity. The applications for the hermetic MEMS packages include accelerometer, gyroscope, resonator, or RF MEMS components.
For the current hermetic MEMS products, the MEMS and sensing element are manufactured in separate substrate. FIG. 1A is a cross-sectional view, illustrating conventional hermetic MEMS device. In FIG. 1A, the conventional hermetic MEMS device includes a lower substrate 100, a middle substrate 102 and a top substrate 104. The lower substrate 100 is patterned to have an indent space 106 and a CMOS circuit with the connection pad 114 as the sensing IC. The middle substrate 102 is fabricated into a MEMS structure with the venting holes 110. The top substrate 104 is fabricated like a cap with the indent space 108. The middle substrate 102 is adhered with the bottom substrate 100 and the top substrate 104 by the adhering rings 112. As a result, the indent spaces 106 and 108 with the venting holes 110 form a hermetic chamber.
However, this conventional process has some drawbacks, including high cost, production yield issue due to three elements being packaged into a device, and parasitic effect for the conjunction of MEMS and sensing IC.
In addition to the hermetic chamber for MEMS device, the mechanical quality in MEMS structure is also important. For the current process with MEMS in integrated circuit (IC) substrate, the MEMS structure is usually formed by the metal layer and dielectric layer used in the interconnection of integrated circuits. However, the mechanical quality is unstable using the interconnection layers of CMOS as MEMS structures. For example, the residue stress may be different significantly within a single wafer or wafer by wafer.
FIG. 1B is a cross-sectional view, illustrating another conventional hermetic MEMS device. In FIG. 1B, there are several device structures formed on the substrate 120, including the MEMS structure 132 and the CMOS circuit 134. In this structure, several metal layers 124-128 are embedded in the dielectric layer 122 to serving as the interconnect while the MEMS structure 132 with polysilicon structure 130 are also formed in the dielectric layer 122. In other words, the MEMS structure 132 is formed in the same process for the interconnect layer of the CMOS circuit 134, usually, the residue stress may be different within a single wafer or wafer by wafer. This cause the mechanical quality being unstable.
When fabricating CMOS MEMS device having the single crystal structure, the fabrication needs to be further concerned and developed.